The first use of the term "piracy" to refer to intellectual property theft — notably, printing another person's book without their permission — dates from the 17th century. As Adrian Johns, author of Piracy: The Intellectual Property Wars from Gutenberg to Gates (University of Chicago Press, 2009), explains:

When and where exactly did people begin to refer to intellectual purloining as piracy? The answer is clearer than one might suppose. It is easy to establish that the usage emerged in English before it did in other European languages. It is more difficult to establish the exact moment the term was coined, but it seems clear that it occurred some time in the mid-seventeenth century. In around 1600 piracy seems not to have carried this meaning at all, except on a few isolated occasions as a metaphor. It appears nowhere in Shakespeare, Ben Jenson, Spenser, Marlowe, or Dekker-or, for that matter, in Francis Bacon, Hobbes, or Milton. This was the first age to see the sustained production of printed dictionaries of English, but the connotation was not mentioned in any of them, whether by Cawdrey (1604), Bullokar (1616), Cockeram (1623), Blount (1656), or Coles (1676). John Donne did once refer to poetic and antiquarian plagiarists as "wit-pyrats" in 1611, and in the early Restoration Samuel Butler likewise called a plagiarist a "wit-caper," a caper being a Dutch privateer. But although these hinted at the later usage, they seem to have been one-off instances. Besides, they addressed not commercial practice, but personal plagiary — a term that itself started to be widely used only around 1600.

At the other end of the century, however, piracy suddenly appears everywhere. It is prominent in the writings of Defoe, Swift, Addison, Gay, Congreve, Ward, and Pope, and pirate suddenly starts to be defined in dictionaries as "one who unjustly prints another person's copy." Very soon after that, it can be seen invoked in learned or medical contentions. In a briefly scandalous case of the 1730s, for example, a physician named Peter Kennedy made the provenance of the term clear when he accused a rival of an attempt to plagiarize his discoveries -- or rather, Kennedy wrote, "to downright pyrate him (as Booksellers call it)." It was a concept that had started as a term of art in the seventeenth-century London book trade, apparently, and was now being appropriated for contests of authorship in other domains. Overall, the evidence for this is unambiguous. And in fact a closer examination indicates that the innovation can be more precisely dated to around 1660-80. At any rate, Donne's seems to be virtually the only example predating the middle of the century, while on the other hand citations start to multiply rapidly in the Restoration. And dictionaries of other European languages published in the late seventeenth and eighteenth centuries then show the term spreading-first to France, then to Italy, and at length to Germany too. Piracy is therefore a legacy of the place and period of the English Revolution, and in particular of the commerce of the book there and then.

The difference, Johns argues, between a "pirate" and a regular thief, in the minds of a 17th-century thinker was the pirates were the "lowest-of-the-low." They lay beyond all society, they lay beyond all honor, they were just the worst of the worst. They had zero honor. They were universally scorned, they were true outlaws, they existed outside of civility itself. (Civility was a major way in which the wealthy and powerful in 17th century England thought about the world; it is at the heart of a lot of debates at the time, including ones that don't seem as obvious to us today, such as "how does one know things." [I have written a bit about this here, in the context of the "Scientific Revolution.")

So this was meant to be a pretty heinous charge — something well beyond simple theft.

The specifics of why that term became used are sort of long and complicated. I will summarize them briefly as just saying that it appears to have been first deployed as part of an argument about whether the English Crown should, as part of the Restoration, reintroduce a system in which it granted absolute monopoly privileges ("patents") over book printing to individuals (as it did before the English Revolution), or whether it should instead be managed by a complicated guild-like system that had come into prominence during the Cromwellian Commonwealth. The details are very specific and niche to this particular economic and political issues at the time, and making sense of the debate at all requires diving pretty deep into what intellectual property looked like in 17th century England and Europe in general (which is very different from how it looks today, as the description of what counted as a "patent" makes kind of clear). The main point, though, is that those in favor of restoring the "patent" privilege of the state basically were arguing that people who printed others' books without permission were pirates, and robbing the state (authors were not really what they were worried about, interestingly enough). The guild on the other end of this debate argued that authorial intellectual property should not be as absolute as the patent system wanted it to be, that the patent system was bad, that they were not pirates, etc. The context is somewhat interesting in that it made a bit more sense to call such acts "piracy" in this way, because it was about an unlawful robbing of the Crown's monies and authority — sort of like poaching on the king's land.

Ironically, perhaps, the fellow arguing that they were "pirates" did win the short-term battle, but lost the war: James II restored the right (after the fellow had died!), but soon after came the Glorious Revolution, and James was out. William and Mary restored the older regime and inshrined the idea that authors had natural rights, which led to a system that looks much more like the modern one. Interestingly enough, the locus of the "piracy" moved from the state to authors in this move, so the term persisted even though the context of its original invocation.

Anyway, I am glossing over quite a bit here, but the main take-away is that the term "piracy" used in this context a) started in English, b) started as a result of very specific debates over printing in Restoration England, c) quickly moved beyond the context of those debates to become a very common charge against people who violated specific commercial or political interests in their non-accord of intellectual property regimes, and d) was from the beginning argued against. For much, much more, the Johns book is essential.